Regulator for xerographic fusing apparatus

ABSTRACT

Electrical fusing apparatus for intermittent operation is controlled by maintaining the apparatus at a bias temperature during standby and by varying the length of its warm up period inversely proportional to the duration of a minimum standby period. During the warm up period, an auxiliary heating element is actuated and a boost power level is employed to accelerate the rise of the fusing apparatus temperature to a desired level for bonding or fusing particles to a support.

United States Patent [191 Klavsons et al.

Feb. 5, 1974 REGULATOR FOR XEROGRAPHIC FUSING APPARATUS Inventors: Uldis Klavsons, Fairport; Thomas B.

Michaels, Pittsford, both of NY.

Assignee: Xerox Corporation, Stamford,

Conn.

Filed: Dec. 23, 1971 Appl. No.: 211,381

US. Cl. 219/216, 432/227 Int. Cl. HOSb l/00 Field of Search 219/216, 388; 263/6 E;

References Cited UNITED STATES PATENTS Tregay et al 219/216 Primary Examiner-C. L. Albritton 7] ABSTRACT Electrical fusing apparatus for intermittent operation is controlled by maintaining the apparatus at a bias temperature during standby and by varying the length of its warm up period inversely proportional to the duration of a minimum standby period. During the warm up period, an auxiliary heating element is actuated and a boost power level is employed to accelerate the rise of the fusing apparatus temperature to a desired level for bonding or fusing particles to a support.

9 Claims, 5 Drawing Figures PAT-ENTEUFEB 51914 snuzmnrs I E T R, ULDIS KLk ysgNg s THOMAS B. MICHAELS ATTORNEY PATENTEDFEB 51914 SilEEIZ-UF'S PATENTEDFEB slam sumunfs PATENTEDFEB 5 I974 llllll'l 'llll- REGULATOR FOR XEROGRAPHIC F USING APPARATUS BACKGROUND OF THE INVENTION This invention relates to xerographic fixing systems and in particular to methods and apparatus for regulating the temperature of fixing systems employing electrically powered heating elements.

In electrophotographic imaging systems, e.g. the xerographic system disclosed in Mayo et a]. US. Pat. No. 3,062,109, an electrostatic latent image is rendered visible or developed by microscopic toner particles that are electrostatically attracted to the latent image. The latent image may be created in several known ways- In xerography, the latent image is formed by charging a xerographic plate including a photoconductive material and by exposing the charged plate to a light image, i.e., a pattern of light and shadow formed from electromagnetic radiation to which the photoconductive material is responsive. The xerographic plate may be reusable in which event the developed toner image is transferred to a suitable support such as ordinary paper. A1- ternately, the xerographic plate may be a coating on a member such as a paper sheet or web in which event the developed toner image does not need to be transferred. In all electrophotographic systems, the toner image is rendered permanent by mechanically fixing the toner to a support. This invention is concerned with thermoplastic and other heat sensitive toner materials that are rendered tacky when heated forming a mechanical bond with the support carrying them. The support may be a plastic member, ordinary paper as in the above reusable xerographic system or paper coated with zinc oxide, cadmium sulfate or other photoconductive material.

One problem with heat fixing systems is that they are not economical to run continuously. Consequently, most electrophotographic systems wherein the image making process is intermittent rather than continuous, the power to the heat fixing system is turned off when the fixing system is not in use. This practice means that the copying process must wait a warm up period before an image can be fused. The warm up period is the time required for the fixing system to reach the temperature capable of softening the toner material. The actual bonding of toner and support is completed when the toner cools to its initial non-tacky or hard condition. This invention is directed toward shortening the warm up period and toward insuring that fixing is not attempted at temperatures above or below the temperature range yielding satisfactory fixing of a toner particle to a support.

Accordingly, it is a principal object of this invention to improve heat fixing systems for electrostatic copying or imaging systems.

Another object of this invention is to shorten the warm up period of intermittently operated heating elements.

Yet another object of the present invention is to insure that the temperature of an electrophotographic heating system repeatedly falls within a defined range when the time between fixing cycles is randomly varied.

Even a further object of the invention is to devise heat regulating means capable of being adjusted to match the warmup and cool-down rates of a particular heat fixing system.

These and other objects of the instant invention are accomplished with regulating method and apparatus that remember the energy level of a heat fixing station and that use elevated energy levels and auxiliary heating elements to accelerate the warmup period of a fixing system. In one embodiment, the fixing system includes six primary electrical heating elements and one auxiliary element. All the heating elements are biased to a standby temperature level to narrow the gap between the on and off temperatures of the fixing system. The rate of warm up is accelerated by temporarily boosting the energy supplied to the heating elements above their steady state level. Separate memory circuits control the length of time that the boosted energy level and auxiliary heating elements are utilized.

DESCRIPTION OF THE DRAWINGS The above and other objects and features of the instant invention will be apparent from the description and the drawings which are:

FIG. 1 is a side elevation, partly in section, of a xerographic imaging machine utilizing a heat fixing system according to the instant invention.

FIG. 2 is a graph illustrating the operation of the timer or memory circuits of the instant invention by displaying the circuit sensing level as a function of time.

FIG. 3 is an electrical schematic diagram of the clock circuits of the instant invention for regulating heat fixing systems.

FIG. 4 is a comparative graph of power versus time for a heatfixing system with and without, respectively, the regulating method and apparatus of the instant invention.

FIG. 5 is a graph of typical image fix levels versus standby time for a xerographic system having a heat fixing system of the type shown in FIG. 1 regulated according to the method and apparatus of the present invention.

DETAILED DESCRIPTION The transfer xerographic system of FIG. 1 has the drum 1 which includes a photoconductive layer coated onto an electrically grounded metal cylinder. The drum as described defines a continuous, reusable xerographic plate or member. The drum is journaled in a frame to rotate in the direction indicated by the arrow to cause the free or image forming surface of the drum to sequentially pass a plurality of xerographic processing stations.

The charging station A includes the corotron 2. Corotron 2, e.g. that described in 'U. S. Pat. No. 2,836,725, is coupled to an appropriate electrical potential and positioned relative to the drum to deposit charge on the free surface of the drum so as to elevate the free surface to a substantially uniform electrical potential, e.g. 800 volts.

The exposure station B includes appropriate lamps 3 and lens 4 mounted to cooperate for a line by line scan of an original placed face down of copyboard or platen 5. The light image created by the scanning of an original is projected onto the free surface of drum 1 through .the aperture 6 in the light step 7. The electrical potenareas absorbing the light in the present positive to positive copying system are referred to herein as the background areas. An example of a background potential is 200 volts when the drum is charged as in the earlier example of 800 volts. The areas of lower potential actually may comprise the image area in a negative to positive copying system. This later copying system is not discussed in detail to avoid redundancy because the present description applies except for logically necessitated changes.

Adjacent the exposure station is the development station C which contains the toner particles for making the latent electrostatic image visible. FIG. 1 shows a cascade development system, by way of example, which includes a motor driven bucket-type conveyor 10. The developer material 1 1 includes carrier particles and toner particles, e.g. thermoplastic resin or other heat softenable materials, and is stored in a sump in the bottom of the housing 11. The buckets scoop up the developer and carry it to the upper portion of the housing where the developer is poured or cascaded over a hopper shute onto the drum 1.

As the developer cascades over the free surface of the drum the toner particles adhere to the electrostatic latent image because of the electric fields associated with the latent image. The toner is electrostatically charged triboelectrically due to a mixing action with the carrier particles. Toner particles consumed during the development process are replenished by a toner dispenser 13 mounted within housing 11.

The pre-transfer station D includes the pre-transfer corotron l and pre-transfer lamp 16. Station D conditions the xerographic plate and toner thereon such that only image area toner particles are transferred to a transfer member 17 at station E. The pre-transfer corotron 15 operation is disclosed in U. S. Pat. No. 3,444,369 to Malinaric.

The transfer station includes, by way of example, means for feeding a transfer member 17 in registration with the toner image on drum 1 and a transfer corotron 18 which charges the backside or non-image carrying side of a transfer member to a high potential, e.g. +2,000 volts for the earlier given examples of +800 and +200 volts at the free surface of drum 1. The electric field established by the charge deposited by corotron 18 and the potentials associated with the drum cause the toner particles in image areas to transfer to member 17.

The transferred toner image is permanently fixed to member 17 at the fixing station F which is the subject of the present invention. Station F includes, by way of example, electrical heating elements 21 that heat soften the toner particles to bond them to the transfer member.

The cleaning station G includes a cleaning corotron 22 and the rotating brush 23 positioned with vacuum housing 24. Corotron 22 is coupled to an alternating potential source to neutralize any non-transferred toner, that is, the corotron charges remaining toner to a near ground potential when the drum is grounded. The brush'sweeps up the remaining particles while the vacuum drawn on the housing 24 pulls the toner into a filter located in box 25.

The cleaning station also includes a lamp for flooding the free surface of the drum with light. Thereafter, the drum once again passes station A and the next image or copy forming cycle begins.

Attention is now directed to the fusing station and the instant invention. The fuser 31 is a radiant heat generating device like that described in Eichler U.S. Pat. No. 2,965,868 and Baker U.S. Pat. No. 3,437,407 the disclosures of which are incorporated herein by reference. Briefly, fuser 31 includes seven electrical heating elements 21and the reflective shield 32 which together direct radiant energy toward the transport mechanism 33. The transport carries the transfer sheets 17 and the transferred toner images to the fixing system or fuser 31. The radiant energy developed by the fuser is absorbed by the opaque toner particles comprising the image and is substantially reflected by the sheet 17 which is usually white in color. The sheet itself may be indirectly heated by fuser 31 through the transport. That is, when there is no sheet passing under the fuser tne transport is exposed to the radiant energy. The transport is a continuous belt so its temperature is raised substantially after several revolutions.

The toner material is rendered tacky or sticky when it absorbs the radiant energy of the fuser. In the tacky state it flows over and/or into sheet 17 depending upon its structure. When sheet 17 is bond paper, the toner flows between the protruding fibers of the paper. The toner cools rapidly when sheet 17 exists the vicinity of fuser 71 and reverts to its initial non-tacky or hardened state. At this time it is intimately bonded or fused with the transfer sheet.

As mentioned at the outset, electrical energy is normally supplied to the heating elements only when copies are to be made. The reason is a matter of dollars and cents. The instant invention regulates the application of electrical energy from source 34 to the heating elements 21 while providing very short warm up periods and means for following the cool down rate of the heating elements so as not to exceed desired fuser temperatures.

The electrical regulating means 35 of the instant invention (FIG. 1) includes bias means for maintaining all the heating elements at a standby temperature when copies are not being made and memory or timer means for regulating various switching operations. The memory means regulates: application of power to the heating elements; the level of the applied power; and the number of heating elements receiving power. The memory means also keeps track of the cool down rate of the heating elements and has two clock circuits whose run lengths can be adjusted to match the cool down rates of different fixing systems.

The foregoing operation is descriptive of fixing system operation after long standby times and not for an initial power on situation. At power on, the regulating circuit 35 is by-passed and the fixing system goes through a 60 second initial heat-up period wherein the heating elements are elevated to the aforementioned bias or standby temperature level.

FIG. 2 is helpful in understanding the operation of the regulating means 35. The y axis 36 represents output voltages in two clock circuits of the type shown in FIG. 3 which are convenient variable parameters for explaining the operation of the regulating means. The x axis represents real time. The switching operations performed by regulating means 35 occur when the voltages in the two circuits arrive at and fall below the sense level voltage 38.

Starting at time zero, the operator presses a print copy" button on the machine to start the generation of two rise curves 39 which are identical for the two clock circuits. The slop of curves 39 are adjustable as will be described. At time zero, power is supplied to all seven heating elements in the fuser and the power level is adjusted to the boost level. The circuit voltages reach the sense level 38 at a predetermined time 40. At time 40, the power is removed from the seventh or auxiliary heating element and the power supplied to the remaining six elements is dropped to a steady state level. The machine, e.g. that in FIG. 1, is ready to permanently fix toner images to sheets 17 at time 40. The six heating elements continue to receive steady state power as long as the machine is producing copies. Time 41 represents the end of the copy production period.

At time 41, clock circuit voltages decay toward the zero or reference voltage level 42 with one circuit voltage following decay curve 43 and the other circuit voltage following decay curve 44. Curve 43 is generated by the clock circuit switching the power level between the boost and steady state levels. Curve 44 is generated by the clock circuit switching the seventh or auxiliary heating element on and off.

5 Curve 43 decays to the reference level within the time period bounded by times 41 and 45 while curve 44 decays to the same level within the period bounded by times 41 and 46. An operator actuation of the print button at time 47 between times 41 and 45 causes each clock circuit to once again start generating the curves 39. In this case, the boost power circuit voltage curve 39a and the auxiliary element circuit voltage curve 39b reach the sense level at different times, i.e., times 50 and 51. The slopes of curves 39a and 39b are identical to curve 39 but they reach the sense level at different times because they start from different levels. The starting levels for curves 39a and 39b are the levels at which decay curves 43 and 44 were when the operator pressed the print button, i.e., time 47. Time 47 to 51 therefore represents the warm-up period for fuser 31. During this warm up period, the boost power level is applied for the full warm up period while the auxiliary element is employed for the shorter period of times 47 to 50.

An operator actuation of the print button at time 55 between times 45 and 46 gives rise to curves 39c and 39d which intersect the sense level at times 56 and 57 respectively. Curve 390 represents the boost voltage circuit and curve 39d represents the auxiliary element circuit which once again have identical slopes to curve 39 but start at different sense levels. In fact, curve 390 is identical to curve 39. Accordingly, the warm up period between times 55 and 57 is identical to the initial warm-up period from time zero to time 40. The boost power level is applied for the full warm up period while the auxiliary element receives the power for the shorter period defined by times 55 and 56.

An operator actuation of the print button at any point after time 57 repeats the described operation being equivalent to time zero in FIG. 2.

As will be described, the clock circuits of FIG. 3 include means for varying the slope of rise curves 39 and of decay curves 43 and 44. This means that these curves can be adjusted to match the warm up and cool down characteristics of fuser 31. It should be noted that the independent regulation of the boost power level and the inclusion of the auxiliary element help regulating means 35 to match the thermo characteristics of a fixing system as well as the changes in the slopes of curves 39, 43, and 44. Furthermore, the clock circuits may also be energizing other elements affecting the operation of the fixing system. For example, in the machine of FIG. 1, a flushing fan positioned in the vicinity of the fuser 31 is turned off, i.e., deenergized, whenever the boost power level is applied.

In the fixing system F of FIG. 1 the maximum warm up period defined by times zero to 40 and times 55 to 57 are about twelve seconds long. The decay curves 43 and 44 fall from sense level 38 to the reference or zero level 42 in about 10 seconds and seconds respectively. These periods are empirically determined to match the needs of a particular system. The need for the system of FIG. 1 may be stated in terms of acceptable fixing for fusing of copy to its transfer member 17. Accordingly, reference is made to the curves of FIG. 4.

The curves 60 and 61 in FIG. 4 represent the taber level of a fixed toner image to 20 pound bond paper of a first copy formed after various standby times. The difference in the two curves 60 and 61 is that the level of the steady state power for curve 60 is greater than that for curve 61. The important aspect of each curve is that they both fall above the minimum acceptable taber level 62. The taber number is a qualitative number ascertained by subjecting a fused or fixed image to standard frictional forces. The flatness of curves 60 and 61 may be adjusted by varying the slopes of curves 39, 43 and 44. In other words, the effectiveness of the regulating circuit 35 can be tested against curves such as curves 60 and 61 to demonstrate that the warm up and cool down characteristics of a fixing system are being properly matched. Naturally, the idea as mentioned at the outset is to keep the warm up period for an intermittently operated fixing system to a minimum. In terms of the xerographic system of FIG. 1, the goal is to shorten the time between an operator pressing a print button and the arrival of the'fused image on sheet 17 into collection tray 29. The ideal flat taber curve is one parallel to and above level 62.

FIG. 5 illustrates the advantage of the standby bias and boost power level of the instant invention. Regulating means 35 maintains a bias power level 65 on all seven heating elements 21 in order to narrow the gap between their standby and operating temperatures. The boost power level 66 is a level above steady state level 65 which is used during the warm up period to accelerate the arrival of the radiant power level to steady state. Curve 68 is the effective electrical power delivered to the heating elements according to the instant invention whereas curve 69 is a like curve for a system not using the bias level or the clock circuits of FIG. 3. The slope of curve 68 is greater than that for 69 because of the bias level and the boost level. The steady state level is reached at time 71 for curve 68 while steady state occurs at a later time 72 for curve 69. Time zero is the instant at which the operator presses the print button while time 73 is equivalent to time 40 in FIG. 2 when the boost power level is switched out and a lower steady state level is switched in.

FIG. 5 represents the radiant power supplied by the fuser 31 not its temperature. With a warm up power curve like curve 68 the temperature of the fuser is elevated from that associated with bias level 65 to a fusing or fixing level within a timer period close to that defined by times zero to 73 but certainly within the period ending at time 71. The temperature rise of fuser 31 is also being assisted by shut off of the flushing fan and by the fact that seven rather than the steady state number of six heating elements are being employed.

The readers attention is now directed to the clock circuit of FIG. 3. Two separate circuits are used and the difference between them needed to obtain the different slopes for curves 43 and 44 will be noted. The heart of the clock circuit is operational amplifier 80 having capacitor 81 coupled in its feedback path and resistor 82 to its input thereby forming an integrator. The resistor network 83 between ground and +V potentials (or other suitable potentials) provide steady state inputs to amplifier 80 which when integrated provide ramp functions such as curves 39, 43 and 44 in FIG. 2. Switch 84 is a switch that is normally in the position shown being coupled to contact 85 but which is thrown to contact 86 when the print button is pressed. Switch 84 automatically returns to contact 85 when the last copy passes under fuser 31. (Switch 84 may be thought of as the print" button.)

The amplifier has its reference terminal 87 referenced to a potential above ground by resistors 90 and 91. When switch 84 is at contact 85 the input to amplifier 80 is positive relative to the reference and is negative when the switch is at contact 86 as is evident from a visual inspection of the circuit. Because of the sign reversal associated with amplifier 80, positive slope curves, e.g. curves 39, are generated at the output 95 when switch 84 is at contact 86 and negative slope curves, e.g. curves 43 and 44, are generated when the switch is at contact 85. Variable resistors 92 and 93 are in series with the amplifier input resistor 82 when the switch is at the two positions. These variable resistors enable the slopes of curves 39 (39a and b), 43 and 44 to be varied.

The output 95 of the integrator 80 is compared to a fixed voltage level at comparator 86. The comparator is also an operational amplifier having its input terminal coupled to the integrator 80 and its reference terminal coupled to a constant voltage level established by resis- 40 tors 90, 91 92 and 97 in the network 83.

The output 98 of the comparator reverse biases diode 99 when the integrator output 95 is below the level at the comparator reference terminal and forward biases the diode when the integrator output is at or above the 45 reference terminal level. The diode is coupled to the base of transistor 100 enabling that device to pass current to the relay 101 when the diode is forward biased. The contact positions of switches controlled by relay 101 are changed when the relay is energized. The switching action performed by one clock circuit cuts out the boost power level and the switching action performed by the other circuit removes power from the auxiliary heating elements. Switches appropriately coupled to the switch 84 cause the boost power and auxiliary element to be cut in upon operator pressing of the print" button.

Modifications to the foregoing described embodiments can be made without departing from the scope of the current invention. Accordingly, any such modifications, alterations or variations are intended to be included herein.

What is claimed is:

l. Fusing apparatus for generating a temperature ca- 6 pable of heating thermoplastic particles to a softened state for bonding the particles to the support carrying them with the bonding temperature being attained within a warmup period following a standby period, said apparatus comprising at least one primary electrical heating element arranged to heat the thermoplastic particles on the support, at least one auxiliary electrical heating element adjacent the primary element for assisting in the heating of the particles and v regulating means coupled to the primary and auxiliary heating elements for controlling the duration of said warmup period in response to the length of the standby period including memory means for regulating independently the actuation of the auxiliary element and the power level applied to the primary and auxiliary elements during the warmup period.

2. Fusing apparatus for generating a temperature capable of heating thermoplastic particles to a softened state for bonding the particles to the support carrying them with the bonding temperature being attained within a warmup period following a standby period, said apparatus comprising at least one primary electrical heating element arranged to heat the thermoplastic particles on the support,

at least one auxiliary electrical heating element adjacent the primary element for assisting in the heating of the particles and regulating means coupled to the primary and auxiliary heating elements for controlling the duration of said warmup period in response to the length of the standby period including memory means for regulating independently the actuation of the auxiliary element and the power level applied to the primary and auxiliary elements during the warmup period, said regulating means includes bias means for maintaining the temperature of the fusing apparatus at a level below the bonding level.

3. Fusing apparatus for generating a temperature capable of heating thermoplastic particles to a softened state for bonding the particles to the support carrying them with the bonding temperature being attained within a warmup period following a standby period, said apparatus comprising at least one primary electrical heating element arranged to heat the thennoplastic particles on the support,

at least one auxiliary electrical heating element adjacent the primary element for assisting in the heating of the particles and regulating means coupled to the primary and auxiliary heating elements for controlling the duration of said warmup period in response to the length of the standby period including memory means for regulating independently the actuation of the auxiliary element and the power level applied to the primary and auxiliary elements during the warmup period, said memory means includes clock means for generating electrical warmup signals that establish the length of the warmup period and for generating electrical decay signals at the start of a standby period that alter the length of the warmup signals when a warmup period is initiated before a minimum standby period expires.

4. The apparatus of claim 3 wherein said clock means includes means for generating at least one warm-up signal for establishing a time period during which a boost power level above a steady state power level is applied to said heating elements and one warm-up signal for establishing a time period during which the auxiliary heating element is actuated.

5. The apparatus of claim 3 wherein said clock means includes means for changing the duration of said warm up and decay signals to adjust the fusing apparatus warm up time to obtain a minimum bonding of particles to their support.

6. Fusing apparatus for generating a temperature capable of heating thermoplastic particles to a softened state for bonding the particles to the support carrying them with the bonding temperature being attained within a warmup period following a standby period, said apparatus comprising,

at least one primary electrical heating element arranged to heat the thermoplastic particles on the support,

at least one auxiliary electrical heating element adjacent the primary element for assisting in the heating of the particles and regulating means coupled to the primary and auxiliary heating elements for controlling the duration of said warmup period in response to the length of the standby period including memory means for regulating independently the actuation of the auxiliary element and the power level applied to the primary and auxiliary elements during the warmup period, said regulating means includes bias means for maintaining the temperature of the fusing apparatus below the bonding temperature during the standby period and wherein said memory means includes first clock means for generating first and second decay periods defining a minimum standby period and first and second rise periods for defining respective periods during which a boost power level above a steady state power level is applied to the heating elements and during which the auxiliary element is actuated said first and second rise periods being shortened inversely to the duration of said first and second decay periods.

7. The apparatus of claim 6 wherein said clock means includes adjustment means to vary said rise and decay periods to obtain optimum bonding of particles to supports after a plurality of standby periods of varied duration.

8. The apparatus of claim 6 wherein said clock means includes electrical circuits for generating linearly increasing voltages comprising said first and second raise signals and linearly decreasing voltages comprising first and second decay signals and wherein the increasing voltages increase from a reference level to a sense level when the warm up period follows a standby period longer than a minimum standby period defined by said decreasing voltages and wherein said increasing voltages increase from levels of said decreasing voltages to the sense level when the warm up period follows a standby period shorter than the minimum standby period.

9. The apparatus of claim 8 wherein said clock means includes an operational amplifier having a capacitor and resistor coupled thereto to function as an integrator for generating said increasing and decreasing voltages. 

1. Fusing apparatus for generating a temperature capable of heating thermoplastic particles to a softened state for bonding the particles to the support carrying them with the bonding temperature being attained within a warmup period following a standby period, said apparatus comprising at least one primary electrical heating element arranged to heat the thermoplastic particles on the support, at least one auxiliary electrical heating element adjacent the primary element for assisting in the heating of the particles and regulating means coupled to the primary and auxiliary heating elements for controlling the duration of said warmup period in response to the length of the standby period including memory means for regulating independently the actuation of the auxiliary element and the power level applied to the primary and auxiliary elements during the warmup period.
 2. Fusing apparatus for generating a temperature capable of heating thermoplastic particles to a softened state for bonding the particles to the support carrying them with the bonding temperature being attained within a warmup period following a standby period, said apparatus comprising at least one primary electrical heating element arranged to heat the thermoplastic particles on the support, at least one auxiliary electrical heating element adjacent the primary element for assisting in the heating of the particles and regulating means coupled to the primary and auxiliary heating elements for controlling the duration of said warmup period in response to the length of the standby period including memory means for regulating independently the actuation of the auxiliary element and the power level applied to the primary and auxiliary elements during the warmup period, said regulating means includes bias means for maintaining the temperature of the fusing apparatus at a level below the bonding level.
 3. Fusing apparatus for generating a temperature capable of heating thermoplastic particles to a softened state for bonding the particles to the support carrying them with the bonding temperature being attained within a warmup period following a standby period, said apparatus comprising at least one primary electrical heating element arranged to heat the thermoplastic particles on the support, at least one auxiliary electrical heating element adjacent the primary element for assisting in the heating of the particles and regulating means coupled to the primary and auxiliary heating elements for controlling the duration of said warmup period in response to the length of the standby period including memory means for regulating independently the actuation of the auxiliary element and the power level applied to the primary and auxiliary elements during the warmup period, said memory means includes clock means for generating electrical warmup signals that establish the length of the warmup period and for generating electrical decay signals at the start of a standby period that alter the length of the warmup signals when a warmup period is initiated before a minimum standby period expires.
 4. The apparatus of claim 3 wherein said clock means includes means for generating at least one warm-up siGnal for establishing a time period during which a boost power level above a steady state power level is applied to said heating elements and one warm-up signal for establishing a time period during which the auxiliary heating element is actuated.
 5. The apparatus of claim 3 wherein said clock means includes means for changing the duration of said warm up and decay signals to adjust the fusing apparatus warm up time to obtain a minimum bonding of particles to their support.
 6. Fusing apparatus for generating a temperature capable of heating thermoplastic particles to a softened state for bonding the particles to the support carrying them with the bonding temperature being attained within a warmup period following a standby period, said apparatus comprising, at least one primary electrical heating element arranged to heat the thermoplastic particles on the support, at least one auxiliary electrical heating element adjacent the primary element for assisting in the heating of the particles and regulating means coupled to the primary and auxiliary heating elements for controlling the duration of said warmup period in response to the length of the standby period including memory means for regulating independently the actuation of the auxiliary element and the power level applied to the primary and auxiliary elements during the warmup period, said regulating means includes bias means for maintaining the temperature of the fusing apparatus below the bonding temperature during the standby period and wherein said memory means includes first clock means for generating first and second decay periods defining a minimum standby period and first and second rise periods for defining respective periods during which a boost power level above a steady state power level is applied to the heating elements and during which the auxiliary element is actuated said first and second rise periods being shortened inversely to the duration of said first and second decay periods.
 7. The apparatus of claim 6 wherein said clock means includes adjustment means to vary said rise and decay periods to obtain optimum bonding of particles to supports after a plurality of standby periods of varied duration.
 8. The apparatus of claim 6 wherein said clock means includes electrical circuits for generating linearly increasing voltages comprising said first and second raise signals and linearly decreasing voltages comprising first and second decay signals and wherein the increasing voltages increase from a reference level to a sense level when the warm up period follows a standby period longer than a minimum standby period defined by said decreasing voltages and wherein said increasing voltages increase from levels of said decreasing voltages to the sense level when the warm up period follows a standby period shorter than the minimum standby period.
 9. The apparatus of claim 8 wherein said clock means includes an operational amplifier having a capacitor and resistor coupled thereto to function as an integrator for generating said increasing and decreasing voltages. 